The circulating fluidized bed boilers require the capture of the particles that are entrained out of a reactor by reactor gases and their separation from the gases in order to recirculate it into said reactor for complete reaction.
The separation is usually made by means of cyclone separators. Cyclonic separation is a method of removing particulates from gases, without the use of filters, through vortex separation. Rotational effects and gravity are used to separate mixtures of solids and gases.
The capture efficiency of the smallest particles is a key issue.
It is known that reducing the diameter of the cyclone separator thus increasing the axial mean velocity in the cyclone body while keeping the same similitude ratio leads to a strong increase of the pressure drop across the cyclone and finally to a decrease of the capture efficiency when trying to keep the pressure drop at a constant limited value by keeping nearly constant the inlet section of the gas-solids entrance into the cyclone. Such types of cyclones having a reduced shape ratio but a nearly constant inlet gas/solids entrance section dimensions to get the same pressure drop as the base cyclone operating at 6 m/s could be qualified as “similar” for the purpose of the comprehension.
Most of the cyclone separators that are currently used in circulating fluidized bed boilers exhibit a good collection efficiency for body mean axial velocity of about 5 to 6 m/s. However, the “similar” cyclone separators, when used at a body mean axial velocity of above 7 m/s lead to a degraded capture efficiency. Thus, it is not possible to use “similar” cyclones if the cyclone collection efficiency of the cyclones operating at 8 m/s is required to be the same as the collection efficiency of the cyclones operating at 6 m/s and some changes have to be invented.
Document U.S. Pat. No. 5,771,844 discloses a cyclone separator able to operate at a body mean axial velocity of 8 m/s with a capture efficiency which is similar to the efficiency of the cyclone separators that are efficient for an axial mean velocity of 6 m/s.
U.S. Pat. No. 5,771,844 discloses that the ratio height/depth of the opening of the cyclone separator inlet is at least 4/1.
This cyclone separator suffers the shortcoming that it makes the inlet duct more costly due to the stiffening needs of the unusually wide vertical dimension. In addition, the cyclone barrel is higher in order to locate the high vertical dimension of the opening. Thus, the cyclone separator body is more expensive than for lower ratios. Moreover, the total height of such a cyclone impacts detrimentally the arrangement of the whole system.